Inbreeding could have helped kill off the Neanderthals, a new study has found, completely overturning a popular theory that our early ancestors had driven the species to extinction.

Researchers at the Eindhoven University of Technology in the Netherlands were surprised by results after they ran simulation models based on existing hunter-gatherer societies, to determine whether factors other than competition from humans could have led to the group being wiped out.

Paleoanthropologists agree that Neanderthals disappeared approximately 40,000 years ago, when anatomically modern humans expanded into Eurasia. However, experts have never fully agreed on just how early modern humans contributed to the Neanderthals’ demise.

For the new study, the researchers simulated model groups of 50, 100, 500, 1,000 and 5,000 individuals. With each group, they looked at the effects of inbreeding, Allee effects (a phenomenon in which the smaller a population, the lower its reproduction rate and vice versa) and random fluctuations of the population due to factors such as births, deaths and sex ratios.

The aim was to see whether such factors could lead to extinction over the course of 10,000 years.

“The mere fact that the Neanderthals were living in such a small population might have been enough to (make them) go extinct,” Krist Vaesen, the head researcher, told the National Post. “It’s not unusual they were living in small populations, either. It’s known that they did.”

Vaesen said external factors such as the ones the researchers studied are quite possibly the reason the Neanderthals don’t exist today — rather than anything modern humans inflicted upon them.

Inbreeding alone, the study found, was unlikely to lead to extinction, with only the smallest studied group of 50 found to have gone extinct in this manner. For groups with 1,000 individuals or fewer, if a quarter or fewer of the females gave birth in a given year, they could also experience extinction, it was found.

Extinction was practically certain for all group sizes when all factors were involved (inbreeding, Allee effects and demographic fluctuations) over the 10,000-year simulation period.

In the study’s conclusion, the authors suggest that the presence of our ancestors likely isolated Neanderthal populations and made them more vulnerable to the study factors before they died out. But humans were not solely to blame, the authors believe.

The Neanderthal man ancestor’s reconstruction, displayed in a show of the Prehistoric Museum in Halle, eastern Germany in July 2004.SEBASTIAN WILLNOW/AFP/Getty Images

“The arrival of (humans) would have been a contributory factor rather than the cause of the extinction,” the authors write.

Vaesen said he was surprised by the results of his study, adding that the researchers themselves had been convinced of the theory that homo sapiens contributed to the total obliteration of the Neanderthals.

The study did not account for random environmental factors, as the models worked with fixed probabilities for mortality, fertility and sex ratios.

“The species’ demise might have been due merely to a stroke of bad, demographic luck,” the authors added.